Photonic Biosensors for Faster Diagnostics

A whispering gallery mode biosensor setup comprising a microchannel containing a fiber-optic filament, a silica microsphere, a laser and a detector within a small device. (Source: NYU Tandon School)

Stephen Arnold and his team at the New York Univer­sity Tandon School of Engi­neering have made a discovery that could lead to Star Trek-like biosensor devices capable of flagging the barest presence in blood of a specific virus or antibody, or protein marker for a specific cancer; or sniffing out airborne chemical warfare agents while they are still far below toxic levels. The discovery follows years of ground­breaking work by Arnold, who in 1995 disco­vered that an optical fiber could excite a whispering gallery mode in polymer micro-beads less than one-third the diameter of a human hair.

Further discoveries and patents led to WGM-bio­sensors capable of gauging the mass of viruses, proteins, and other nano­particles by sending them into space­craft-like orbit around the micro-bead, thanks to a photonic “tractor beam” caused by the reso­nating light. Arnold and colla­borators then devised a way to make these WGM-bio­sensors sensitive enough to identify even the smallest individual bio-particles from the RNA virus MS2 to single molecules down to 6 zepto-grams, below the mass of all known cancer markers. Many companies, including Genalyte, employ WGM-bio­sensors in diagnostic products that can perform dozens of bio­assays in minutes.

Now, Arnold and his team are the first to find a way to deter­mine the density of charges on an area of a WGM micro-bead’s surface, as well as the charge of an ensnared nano­particle or virus, by measuring how light frequency fluctuates as the tiny particle follows its wobbly course around the sphere. This disco­very could allow researchers and manu­facturers not just to identify nano­particles, but to mani­pulate them. The WGM-biosensor is a device the size of a small smart­phone comprising a tunable laser guided down a specially treated fiber optic fila­ment with a detector at the far end of the fila­ment measuring the light’s inten­sity and reso­nance. A tiny silica bead next to the filament diverts a portion of the light beam, which begins to reso­nate within the bead.

While the WGM-bio­sensor’s ability to identify indi­vidual nano­particles led to highly sensitive measuring capa­bilities, Arnold’s latest disco­very could make possible bio­sensors tailored to very specific applications, from wearable sensors for soldiers and rescuers designed to detect extremely low concen­trations of a suspected airborne nerve agent, to ways of increa­sing the effi­ciency of nano­particle drug uptake and redis­tribution. “Charge controls the ability to transport particles that are inter­acting with cells and other objects that possess electric fields,” he said. “By deter­mining the charge of a virus, for example, you can under­stand how it can be transported to the cell surface. You need to under­stand this mechanism in order to engineer a WGM micro-bead that has a specific antigen at a specific region of its surface so that the biosensor can attract specific patho­gens or other biomo­lecules.”

Arnold and his team were able to extract the electro­static force between the orbiting nano­particle and the surface of the glass bead through experiments based on the obser­vation that the nano-orbital pheno­menon requires a near balance between the electro­static force and the known optical tractor beam force, just as a weighing scale balances the force of a spring against your body’s weight.

“The difference in the strength of the force being measured is extra­ordinarily small,” said Arnold. “With this force in hand both the charge on the nano­particle and the micro­cavity charge density could be calcu­lated through a series of experi­ments.” The team next plans to use the disco­very to develop tech­nology for “photonic printing,” the ability to quickly create numerous task-specific WGM-bio­sensors, with specific mole­cules attached to specific areas of the micro-bead. (Source: NYU Tandon)

Reference: J. R. Lopez et al.: Whispering gallery mode coulometry of the nanoparticle-microcavity interaction in aqueous solution, Appl. Phys. Lett. 112, 051109 (2018); DOI: 10.1063/1.5017041

Link: Microparticle Photophysics Laboratory for BioPhotonics, NYU Tandon School of Engineering, New York, USA

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